1. Field of the Invention
This invention relates to a method and equipment for reducing contaminants such as volatile organic compounds (VOC's) and carbon monoxide (CO) normally present in dryer offgas that is discharged into the atmosphere from a moist organic product drying process. The equipment includes a product pre-dryer/waste heat evaporator, a primary product dryer, thermal oxidizing apparatus, a furnace, which serves to deliver hot products of combustion to the thermal oxidizing apparatus, and a gas-to-gas heat exchanger of the indirect type having a hot gas side and a cool gas side, hereinafter referred to as the primary heat exchanger, for bringing the hot gaseous output from the thermal oxidizing apparatus that is ultimately discharged into the atmosphere into indirect heat exchange relationship with recycle dryer offgas to increase the temperature of the recycle dryer offgas prior to its reentry into the dryer.
Efficient thermal oxidation of VOC's and CO requires correlation of four factors occurring simultaneously:
1) Adequate temperature;
2) Adequate oxygen concentration;
3) Adequate residence time; and
4) Adequate turbulence.
In the present process, a rotary waste heat evaporator is utilized to remove moisture from a portion of the dryer offgas thereby allowing the thermal oxidizing apparatus to achieve a much higher temperature while maintaining an adequate oxygen concentration than compared to conventional processes. The amount of moisture removed from the dryer off gas by the rotary waste heat evaporator and the input of fuel to the furnace are controlled and adjusted to provide a hot gaseous output from the thermal oxidizing apparatus that is at a temperature of at least about 1600° F. with an optimum 5% oxygen content by volume, which are sufficiently high to substantially oxidize VOC's and CO in dryer offgas that is discharged into the atmosphere.
2. Description of the Prior Art
Dryers have been used for many years to lower the moisture content of a variety of organic products, such as grain, including distiller's grain and the like, which nominally may have a water content as high as 60-75%. The recent emergence of ethanol plants producing substantial quantities of moist distiller's grain as output residue requiring drying for further commercial use, has rekindled interest in more efficient drying processes while, at the same time, necessitating that dryer offgas discharged into the atmosphere contain reduced amounts of VOC's and CO.
Commercial drying equipment has been previously designed and constructed to dry organic products to a predetermined acceptable level, which is normally about 10% moisture by weight, wet basis. It has been known for some time to incorporate thermal oxidizing apparatus in processes and equipment for drying moist organic products in order to lower the VOC and CO content of the product output from the dryer. In order to reduce the VOC and CO content of dryer offgas introduced into the atmosphere employing a thermal oxidizer, the hot gaseous output from the oxidizer should be at least about 1600° F. and the oxygen concentration should be at least about 5% by volume. Heretofore, the temperature of the output from the thermal oxidizer has been limited to temperatures in the order of 1400° F. when the oxygen concentration is increased to 5% by volume; hence, VOC and CO reduction has not been optimum.
Even though residence time of the offgas being oxidized was not restricted and gas turbulence not a significant factor, it was not heretofore feasible to adequately control both the temperature of the thermal oxidizer and its oxygen concentration, in order to significantly lower the VOC and CO content of the offgas introduced into the atmosphere. The temperature and the oxygen concentration could be controlled individually, but not simultaneously for most efficient operation of the thermal oxidizing apparatus.